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Tool Talk Discussion Forum

3 phase converter

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01-31-2014 10:36:46

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Hi all, I just bought a new toy (to me) I got deal on a milling machine so now it sits in my shop. What is the best way to power it, VFD or a static converter. Right now I cant afford a rotary converter so I"m looking for the least expensive way to get it running. The one part I didn"t expect is the a 3 phase power feed. Can anyone tell me if I can use a VFD to power the main drive and also run the powerfeed at the same time with it?? Will they take me switching the power feed on and off while running the main motor?

Thanks for all your great advice


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baldwin racing

02-03-2014 05:25:01

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  
rotory phase converters are not to bad priced now a days $370 free shipping for either a 5 hp or 3 hp can not remember....either way it would run your mill..look on ebay there is a couple companies selling them on there with pretty good warranty....

I received a rotory phase converter when I bought my mill...I changed out the old caps and also installed a start cap....was pull just push a start button....

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02-02-2014 21:04:28

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  

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02-01-2014 06:43:41

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  
Forgot to add, I go to Homeshopmachinist and hobbymachinist which I think are very good sites. You won't get slammed for not being a professional or asking simple questions. They have some very good people. Google them for the right spelling.

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02-01-2014 06:26:36

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  
Ihave a 3hp lathe with a Phase-A-Matic on it. It works fine for my use. I bought a 3hp mill last year and I had a 5hp old old motor off an air compressor that I bought and put a single phase on. I bought a rotary comverter kit on Ebay for $125. Push button start. Works fine but I think the quality of the kit is a little short. If I hadn't been in a hurry, I would have scrounged up the parts myself and maybe saved some money. My mill is a variable speed and the power feed is 120v.

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Stephen Newell

02-01-2014 03:23:47

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  
While on the topic of phase converters, I have always wondered why it it recommended to use a higher HP motor on the idler motor on a rotary converter than the machine you are operating?

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02-02-2014 22:18:16

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 Re: 3 phase converter in reply to Stephen Newell, 02-01-2014 03:23:47  
Re-reading these posts about HM converters- it may be a matter of semantics- when I refer to the idler motor, I mean the one that spins the bigger 3 phase motor to get it running. Sounds like you and others may refer to the big one as the idler. A commercial rotary phase converter is just like a motor, without a shaft sticking out, and it"s controlled and started by built in capacitors.

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Stephen Newell

02-03-2014 03:56:29

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 Re: 3 phase converter in reply to JMS/.MN, 02-02-2014 22:18:16  
Yes that is the one I meant. It just seems to me that motor has more to do with dividing the phase to 120 degrees and I would like to understand why the HP matters.

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02-03-2014 07:20:11

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 Re: 3 phase converter in reply to Stephen Newell, 02-03-2014 03:56:29  
Quoting Removed, click Modern View to see

Wouldn't the idler windings still need to handle the amps to power the 3 ph equipment.

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Dusty MI

02-03-2014 16:22:49

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 Re: 3 phase converter in reply to sflem849, 02-03-2014 07:20:11  
Quoting Removed, click Modern View to see


I don't remember for sure but I think the idler motor needs to be 125% larger than the machine motor. Or if you are going run more than 1 three phase motor at a time, it needs to be 125% larger than the largest 3ph motor.
The more 3ph motors run at the same time the better the 3ph gets.

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02-01-2014 19:32:48

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 Re: 3 phase converter in reply to Stephen Newell, 02-01-2014 03:23:47  
With a homemade converter, the idler motor is small- anything big enough to spin the 3 phase motor used to make the converter. The converter motor needs to be bigger than the 3ph on the machine tool in order to develop enough power, as mentioned in another post.

I made mine with a 5 hp 3 phase, spun by a half horse single phase motor. It runs a 3 hp 3 phase turning lathe and a 3 phase Milwaukee mill. I had the motors and I think both switch boxes, just had to buy some cords and plug-ins. google HM phase converters for wiring diagrams and examples.

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01-31-2014 20:27:09

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  
Depends on who you ask and what the circumstances are as to what is "best".

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01-31-2014 18:45:20

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  
You said best then cheapest? which do you want?
Least expensive way? Wire switched single phase 220 to any two of the three input lines on the main motor. Spin the motor in the direction you want it to run by what ever means is cheapest for you. I have seen single phase motors belted up, lawn mower recoil starters adapted and used, electric drills. I have seen rope just wrapped around the shaft. I have seen one that had the fan shroud removed and they stuck a screwdriver in the fan and spun it up. You dont have to get it up to speed just going the right direction as long as there isn't any load dragging on it. By what ever means you choose get it spinning and then throw the switch. The motor will run and you will have near 2/3 name plate HP. 3 phase for the power feeds will come from the same single phase you just attached to the main motor and the third phase will come from the un-powered third input connection for the main motor, the voltage being induced in the main motor just like the idler motor in a rotary set up.

VFDS are great where they fit but have their own set of problems too. Old motors dont like them. I found that out after ruining an old U frame 1100 RPM 10HP motor with one. Motor shop guy said something about voltage spikes and old type insulation. They also ramp up quickly in price once you get past a couple HP. I don't hate VFDs, but they are not cure-alls. You can walk your way into a rotary by building it yourself. up to 10HP converters/starters are on Ebay right now for $99, look up Phase Craft, the are made in USA. Use it to start your mill and if you decide later you need some extra power then buy an idler motor.

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01-31-2014 17:53:25

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  
If you"ve got a mill like mine where the drive and feed motors are all 3 phase then you can"t use a static converter as they will handle only one motor at a time. A far as a VFD is concerned I have heard the same about them but can"t say for sure.

That said a rotary converter isn"t that hard to build if you take a little time to search the web and find plans for one. Between Craigslist and a relative that gave me some old motor contactors and capacitors, I built one using a brand new 10HP Teco-Westinghouse motor and the mentioned capacitors, for less than $250. I run a Supermax mill with it that has a 5 HP main drive, the feed motor, and the coolant pump, all with no problems.

That said if you find a company that does work on municipal wells, etc (like the place Dad works) they will often pull "blown up" pump controllers out and simply replace them -vs- trying to repair them. Usually even the small ones contain at least one motor starter and a start and run capacitors. In other words you should be able to get pretty much everything needed from them, relatively cheap, if you ask, just like I did. An alternative is a salvage yard, etc that takes in industrial equipment with electrical cabinets full of motor controllers, which are the expensive items needed. The capacitors are relatively cheap from places like Surplus Center, etc.

In the end, a little research and a few hours putting parts together, and you can have a rotary converter that would ordinarily cost you $800 for not much than the cost of a good, new/used three phase motor.

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01-31-2014 14:53:32

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  
Yes, you can use a VFD to run the motor. There are models that use single phase input and 3 phase output.

Some are rated to do this, but I hear you can use a 3 phase input, and only use single phase in, but derate it 58%. Also consider, your mill is either a 1 1/2 or 2 HP. Odds are you will never use all that power, so a 2 HP unit will suit you well.

You will want to control the motor start/stop/reverse with the VFD, not the drum switch that's on it now. The drum switch will KILL a VFD!

The VFD's have gotten cheaper, and more available, as in Ebay, Craigs list, Etc. If you go new, look at Westinghouse Teco brand, good converts, good price.

That will be a much easier install than a rotary converter.

The power feed runs on 120v, or all I've ever seen do.

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Mike M

01-31-2014 11:36:52

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  
I have a converter box bought many years ago from wholesale tool. It has worked fine. The VFD are now around and priced pretty good. Very nice to have that speed selection. They are kind of slow to react when turning on and off.

Used an old 3 phase motor to power the mill before got the converter box.

Just get the motor spinning flip on the power then power up the 3 phase. I just put a pulley on it and used a pull rope wrapped around it.

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01-31-2014 11:05:26

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 Re: 3 phase converter in reply to teachingscience, 01-31-2014 10:36:46  
Power feed on table usually is run with 120V, separate from the spindle motor. I haven't run a mill using a VFD yet, but I have looked into it. Advantage is that the speed can be controlled instantly without changing pulley size. Some info here -->

I run my lathe and Bridgeport using a rotary converter. I bought the control box from Anderson Converters 3HP panel is $169, then just add a 3HP 3phase motor as an idler. Very simple to wire, and works good. I just bought a 10HP panel from them to set up another converter for a larger lathe and surface grinder.

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